service/ccm: strip Accept-Encoding before forwarding to avoid untracked usage

When clients (e.g. Node.js Anthropic SDK) explicitly set Accept-Encoding: gzip,
Go's http.Transport does not transparently decompress the response body, because
it only does so when it added the header itself. This causes CCM's json.Unmarshal
to receive raw gzip bytes, silently failing to parse usage data and leaving the
usage counter unchanged.

Fix: remove Accept-Encoding from the outgoing proxy request. Transport adds it
automatically and transparently decompresses response.Body before CCM reads it.

Wire compression (CCM→Anthropic) is preserved — Transport still negotiates gzip.
Only CCM→localhost path is affected; compression on loopback has no practical
benefit.

service/ccm/service.go

@@ -362,6 +362,13 @@ func (s *Service) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
+	serviceOverridesAcceptEncoding := len(s.httpHeaders.Values("Accept-Encoding")) > 0
+	if s.usageTracker != nil && !serviceOverridesAcceptEncoding {
+		// Strip Accept-Encoding so Go Transport adds it automatically
+		// and transparently decompresses the response for correct usage counting.
+		proxyRequest.Header.Del("Accept-Encoding")
+	}
+
 	anthropicBetaHeader := proxyRequest.Header.Get("anthropic-beta")
 	if anthropicBetaHeader != "" {
 		proxyRequest.Header.Set("anthropic-beta", anthropicBetaOAuthValue+","+anthropicBetaHeader)